It took me a bit of time to extract this info from the docs and the board itself, so I thought I'd post it here so any searchers in the future will get there quicker.
This is for the Duemilanove
//Using Port C as digital output
In your setup: // Turn analog inputs to digital outputs DDRC=0xFF ; // These will be pin numbers 14 .. 19
In your loop: digitalWrite (14, HIGH); // ...etc... digitalWrite (19, LOW);